The art of lithographic printing is based upon the immiscibility of oil and water, in which oily material or ink is preferentially retained by the image areas and the water or fountain solution is preferentially retained by the non-image areas of the printing plate. When a suitably prepared surface is moistened with water and ink is applied, the background or non-image areas retain the water and repel the ink while the image areas accept the ink and repel the water. The ink on the image areas is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and plastics. Commonly, the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Lithographic printing plates can be either negative-working or positive-working, and comprise one or more radiation-sensitive layers on a suitable substrate, such as metal or polymeric support. The radiation-sensitive layer generally includes one or more radiation-sensitive components that may be dispersed in a suitable binder. Alternatively, the radiation-sensitive component can also be the binder material. Certain useful printing plates can be used either as positive-working or negative-working.
A negative-working printing plate generally has a light sensitive layer composed of a radiation-sensitive component such as an unsaturated resin on a suitable substrate. Upon exposure to light, the exposed areas are hardened, leaving non-exposed areas removable during development. Certain negative-working printing plates contain novolac resins, a cross-linking agent, and a radiation-sensitive component that produces acid on exposure. By subsequently heating the plate, only the exposed areas are cured and the unexposed areas can be removed by a developer. The exposed, hardened areas are therefore oleophilic and will accept ink while the non-exposed underlying areas of the substrate are hydrophilic.
An alkaline developable positive-working printing plate generally has a light sensitive layer comprising a novolac resin and a radiation-sensitive component such as an o-diazoquinone or diazonaphthoquinone compound. Upon exposure to light the radiation-sensitive component is converted to the corresponding carboxylic acid. The use of an alkaline developer will remove only the exposed areas of the radiation-sensitive layer, leaving the surface of the support. Because the surface of the support is hydrophilic, the uncovered non-image area attracts water and repels the oily ink. The image area remaining after development is oleophilic thereby repelling water and attracting the printing ink.
Various aqueous solutions are known for use as developers for both positive-working and negative-working printing plates. For example, Yamasue, U.S. Pat. No. 4,259,434, describes use of a dispersion of a silicate to develop positive-working printing plates. These solutions include alkali metal silicates, in which the ratio of SiO.sub.2 to M is from 0.5 to 0.75 ("M" being the alkali metal) at 1-4% SiO.sub.2 concentration. However, many of such developers are overly active and attack or remove the unexposed image on the plates. Further, such developers attack the aluminum oxide layer and the aluminum on the back of the printing plate to such an extent that the developer activity decreases considerably and that sludging problems occur in the developing processor. The decrease in the developer activity due to its reaction with the carbon dioxide in air is significant as well. The replenishment rate of such developers is critical because the operational range of the developers is very narrow.
Seino, U.S. Pat. No. 4,452,880 describes silicate-containing developers in which the SiO.sub.2 to alkali metal oxide ratios are much higher, that is between 1.6 and 2.0, and the % SiO.sub.2 concentration is from 2 to 9%. These developers have relatively low activity, resulting in slow or incomplete development within the time suitable for practical commercial use. Thus, higher amounts of silicate must be included. Such higher amounts can be disadvantageous due to cost, residue on the plates and the potential for sludging in the processing equipment.
Miller, U.S. Pat. No. 5,851,735 discloses an aqueous alkaline developing composition that is useful in developing either positive-working or negative-working lithographic printing plates. The composition has a pH of at least 12 and comprises an alkali metal silicate and at least 6 wt % of a water-soluble or water-dispersible thickener, such as glycerol.
Toyama, GB-A-2,276,729 (DE-A-4 411 176) discloses a developer or replenisher for both positive-working and negative-working lithographic printing plates comprising an alkali metal silicate and a water-soluble ethylene oxide adduct, obtained by addition of ethylene oxide to a sugar alcohol having not less than 4 hydroxy groups. This developer is stable and shows a high developing efficiency.
Thus, a need exists for a developer system for both positive-working plates and negative-working alkaline developable plates including thermal plates, which can be used in one processor. The developer system should be able to develop both positive- and negative-working plates with a high throughput, should have a small overflow rate, should not produce any sludge, should not attack the aluminum or alumina of the printing plate base, should not attack the coating particularly in case of positive-working plates and the control of the developer activity should be carried out by means of conductivity measurements, because especially for thermal plates there are no other control elements available.